1. Field of the Invention
The present invention is related to a multi-frequency hidden antenna for mobile phones; and especially to a planar miniaturized antenna of such kind suitable for using as a multi-frequency transceiver antenna provided in a communication equipment such as a mobile phone.
2. Description of the Prior Art
As the mobile phones were emerged in the markets in early days, most of them used exposed coils of spiral structure as the main elements of antennas. These coil antennas used widely nowadays are divided into two major typesxe2x80x94contractible type and fixed type. No matter what type of structure is used, under normal circumstances, it still protrudes with a specific length out of the body""s top surface of a mobile phone. The conventional coil antennas become more and more unsuitable for the design requirement of the miniaturized mobile phones, hence microstrip antennas have been developed. Because the characteristics of the microstrip antennas are flatness and small space demanding, such antennas certainly will become the mainstream products for the miniaturized mobile phones.
A microstrip antenna of the early stage, as disclosed in U.S. Pat. Nos. 3,921,177 or 3,810,183, usually consists of a round or rectangular thin metal sheet, and dielectric substance is stuffed between it and the ground; but these microstrip antennas are only compatible with narrower bandwidths. Taiwan Patent No. 81108896 (U.S. patent application Ser. No. 07/798,700) provided a microstrip antenna with diminished size and broadband, however the defects of this kind of antenna are to install the spiral antenna elements on separate ground boards, and to stuff dielectric and loading material of specific thickness between them. The size of the whole antenna was still hard to be further reduced.
Among modern planar inverted F-antennas (PIFA), dual-frequency antennas (IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL 45, NO. 10, OCTOBER 1997) are of an ideal type of miniaturized microstrip antenna, however, by the fact that:
Electrical volume of an antenna/frequency bandxc3x97gainxc3x97efficiency=a constant,
So long as the antenna is made planar and miniaturized, its bandwidth and efficiency of radiation will be reduced and will be necessary to be improved.
The object of the present invention is to provide a multi-frequency hidden antenna for mobile phones, of which the electric supplying line adopts a shortened single-pole antenna of xc2xcxcex, the open circuit end thereof is in the shape of an inverted triangle in order to get a higher gain, and the single-pole antenna of xc2xcxcex is flattened to obtain the desired impedance matching, a short circuit line is provided near the electric supplying point. The short circuit portion is like a loaded capacitor, its harmonic oscillation frequency can be adjusted by changing of its position to get a multi-frequency effect, and thus a planar and miniaturized multi-frequency antenna formed from a coplanar waveguide (CPW) antenna and a planar inverted F-antenna (PIFA) is obtained.